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Cosmology, Largescale Structure of the Universe and Galaxy Formation
 Numerical Cosmology (Nbody and Hydrodynamics)
 Dark Matter and Dark Energy (phenomenology)
 Cosmic reionization
 Peculiar velocity & nonGR effects
 Cosmic topology



Publication at kias
NUMBER  
AUTHOR  Kim, Juhan 
TITLE  Living with Neighbors. IV. Dissecting the SpinOrbit Alignment of Dark Matter Halos: Interacting Neighbors and the Local Largescale Structure 
ARCHIVE  
FILE  
JOURNAL  ASTROPHYSICAL JOURNAL, 2021 
ABSTRACT  Spinorbit alignment (SOA; i.e., the vector alignment between the halo spin and the orbital angular momentum of neighboring halos) provides an important clue to how galactic angular momenta develop. For this study, we extract virialradiuswise contact halo pairs with mass ratios between 1/10 and 10 from a set of cosmological Nbody simulations. In the spinorbit angle distribution, we find a significant SOA in that 52.7% +/ 0.2% of neighbors are on the prograde orbit. The SOA of our sample is mainly driven by lowmass target halos (h (1) M (circle dot)) with close merging neighbors, corroborating the notion that tidal interaction is one of the physical origins of SOA. We also examine the correlation of SOA with the adjacent filament and find that halos closer to the filament show stronger SOA. Most interestingly, we discover for the first time that halos with the spin parallel to the filament experience most frequently prograde polarinteraction (i.e., fairly perpendicular but still prograde interaction; spinorbit angle similar to 70 degrees). This instantly invokes the spinflip event and the progradepolar interaction will soon flip the spin of the halo to align it with the neighbors orbital angular momentum. We propose that SOA originates from the local cosmic flow along the anisotropic largescale structure, especially that along the filament, and grows further by interactions with neighbors. 

Publication at kias
NUMBER  
AUTHOR  Kim, Juhan 
TITLE  Revealing the Local Cosmic Web from Galaxies by Deep Learning 
ARCHIVE  
FILE  
JOURNAL  ASTROPHYSICAL JOURNAL, 2021 
ABSTRACT  A total of 80% of the matter in the universe is in the form of dark matter that composes the skeleton of the largescale structure called the cosmic web. As the cosmic web dictates the motion of all matter in galaxies and intergalactic media through gravity, knowing the distribution of dark matter is essential for studying the largescale structure. However, the cosmic webs detailed structure is unknown because it is dominated by dark matter and warmhot intergalactic media, both of which are hard to trace. Here we show that we can reconstruct the cosmic web from the galaxy distribution using the convolutionalneuralnetworkbased deeplearning algorithm. We find the mapping between the position and velocity of galaxies and the cosmic web using the results of the stateoftheart cosmological galaxy simulations of IllustrisTNG. We confirm the mapping by applying it to the EAGLE simulation. Finally, using the local galaxy sample from Cosmicflows3, we find the dark matter map in the local universe. We anticipate that the local dark matter map will illuminate the studies of the nature of dark matter and the formation and evolution of the Local Group. Highresolution simulations and precise distance measurements to local galaxies will improve the accuracy of the dark matter map. 

Publication at kias
NUMBER  K21006 
AUTHOR  Park, Changbom,Kim, Juhan,Appleby, Stephen,Appleby, Stephen 
TITLE  Cosmological Parameter Estimation from the Twodimensional Genus TopologyMeasuring the Expansion History Using the Genus Amplitude as a Standard Ruler 
ARCHIVE  arXiv:2102.01365 
FILE  
JOURNAL  ASTROPHYSICAL JOURNAL, 2021 
ABSTRACT  We measure the genus of the galaxy distribution in twodimensional slices of the SDSSIII BOSS catalog to constrain the cosmological parameters governing the expansion history of the universe. The BOSS catalogs are divided into 12 concentric shells over the redshift range 0.25 z < 0.6, and we repeatedly measure the genus from the twodimensional galaxy density fields, each time varying the cosmological parameters used to infer the distanceredshift relation to the shells. We also indirectly reconstruct the twodimensional genus amplitude using the threedimensional genus measured from SDSS Main Galaxy Sample with galaxies at low redshift z < 0.12. We combine the low and highredshift measurements, finding the cosmological model that minimizes the redshift evolution of the genus amplitude, using the fact that this quantity should be conserved. Being a distance measure, the test is sensitive to the matter density parameter (Omega(m)) and equation of state of dark energy (w(de)). We find a constraint of w(de) = 1.05(0.13)(0.12), Omega(m).=.0.303.+/.0.036 after combining the high and lowredshift measurements and combining with Planck CMB data. Higherredshift data and combining data sets at low redshift will allow for stronger constraints. 

Publication at kias
NUMBER  K21007 
AUTHOR  Park, Changbom,Shim, Junsup,Kim, Juhan 
TITLE  Identification of Cosmic Voids as Massive Cluster Counterparts 
ARCHIVE  arXiv:2012.03511 
FILE  
JOURNAL  ASTROPHYSICAL JOURNAL, 2021 
ABSTRACT  We develop a method to identify cosmic voids from the matter density field by adopting a physically motivated concept that voids are the counterpart of massive clusters. To prove the concept we use a pair of ?CDM simulations, a reference and its initial densityinverted mirror simulation, and study the relation between the effective size of voids and the mass of corresponding clusters. Galaxy clusterscale dark matter halos are identified in the Mirror simulation at z = 0 by linking dark matter particles. The void corresponding to each cluster is defined in the Reference simulation as the region occupied by the member particles of the cluster. We study the voids corresponding to the halos more massive than 10(13) h(1) M. We find a powerlaw scaling relation between the void size and the corresponding cluster mass. Voids with a corresponding cluster mass above 10(15) h(1) M occupy similar to 1% of the total simulated volume, whereas this fraction increases to similar to 54% for voids with a corresponding cluster mass above 10(13) h(1) M. It is also found that the density profile of the identified voids follows a universal functional form. Based on these findings, we propose a method to identify clustercounterpart voids directly from the matter density field without their mirror information by utilizing three parameters such as the smoothing scale, density threshold, and minimum core fraction. We recover voids corresponding to clusters more massive than 3 x 10(14) h(1) M at a 70%74% level of completeness and reliability. Our results suggest that we are able to identify voids in a way to associate them with clusters of a particular mass scale. 

Publication at kias
NUMBER  P20053 
AUTHOR  Park, Changbom,Kim, Yonghwi,Lee, Jaehyun,Lee, Jaehyun,Kim, Juhan,Pichon, Christophe 
TITLE  The Horizon Run 5 Cosmological Hydrodynamical Simulation: Probing Galaxy Formation from Kilo to Gigaparsec Scales 
ARCHIVE  https://arxiv.org/abs/2006.01039 
FILE  
JOURNAL  ASTROPHYSICAL JOURNAL, 2021 
ABSTRACT  Horizon Run 5 (HR5) is a cosmological hydrodynamical simulation that captures the properties of the universe on a Gpc scale while achieving a resolution of 1 kpc. Inside the simulation box, we zoom in on a highresolution cuboid region with a volume of 1049 x 119 x 127 cMpc(3). The subgrid physics chosen to model galaxy formation includes radiative heating/cooling, UV background, star formation, supernova feedback, chemical evolution tracking the enrichment of oxygen and iron, the growth of supermassive black holes, and feedback from active galactic nuclei in the form of a dual jetheating mode. For this simulation, we implemented a hybrid MPIOpenMP version of RAMSES, specifically targeted for modern manycore manythread parallel architectures. In addition to the traditional simulation snapshots, lightcone data were generated on the fly. For the postprocessing, we extended the friendsoffriend algorithm and developed a new galaxy finder PGalF to analyze the outputs of HR5. The simulation successfully reproduces observations, such as the cosmic star formation history and connectivity of galaxy distribution, We identify cosmological structures at a wide range of scales, from filaments with a length of several cMpc, to voids with a radius of similar to 100 cMpc. The simulation also indicates that hydrodynamical effects on small scales impact galaxy clustering up to very large scales near and beyond the baryonic acoustic oscillation scale. Hence, caution should be taken when using that scale as a cosmic standard ruler: one needs to carefully understand the corresponding biases. The simulation is expected to be an invaluable asset for the interpretation of upcoming deep surveys of the universe. 

Publication at kias
NUMBER  P20022 
AUTHOR  Park, Changbom,Zheng, Yi,Kim, Juhan,Park, Hyunbae,Tonegawa, Motonari 
TITLE  Cosmological Information from the Smallscale Redshiftspace Distortion 
ARCHIVE  https://arxiv.org/abs/2005.12159 
FILE  
JOURNAL  ASTROPHYSICAL JOURNAL, 2020 
ABSTRACT  The redshiftspace distortion (RSD) in the observed distribution of galaxies is known as a powerful probe of cosmology. Observations of largescale RSD, caused by the coherent gravitational infall of galaxies, have given tight constraints on the linear growth rate of the largescale structures in the universe. On the other hand, the smallscale RSD, caused by galaxyrandom motions inside clusters, has not been much used in cosmology, but it also has cosmological information because universes with different cosmological parameters have different halo mass functions and virialized velocities. We focus on the projected correlation function w(r(p)) and the multipole moments xi(l) on small scales (1.430 h(1) Mpc). Using simulated galaxy samples generated from a physically motivated most bound particle (MBP)galaxy correspondence scheme in the Multiverse Simulation, we examine the dependence of the smallscale RSD on the cosmological matter density parameter Omega(m); the satellite velocity bias with respect to MBPs, b(nu)(s) and the merger timescale parameter alpha. We find that alpha = 1.5 gives an excellent fit to the w(r(p)) and xi(l) measured from the Sloan Digital Sky SurveyKorea Institute for Advanced Study valueadded galaxy catalog. We also define the strength of the Fingers of God as the ratio of the parallel and perpendicular size of the contour in the twopoint correlation function set by a specific threshold value and show that the strength parameter helps constrain (Omega(m), b(nu)(s), alpha) by breaking the degeneracy among them. The resulting parameter values from all measurements are (Omega(m), b(nu)(s)) = (0.272 +/ 0.013, 0.982 +/ 0.040), indicating a slight reduction of satellite galaxy velocity relative to the MBP. However, considering that the average MBP speed inside halos is 0.94 times the dark matter velocity dispersion, the main drivers behind the galaxy velocity bias are gravitational interactions, rather than baryonic effects. 

Publication at kias
NUMBER  Q20006 
AUTHOR  Park, Changbom,Kim, Juhan,Appleby, Stephen,Appleby, Stephen 
TITLE  Cosmological Parameter Estimation from the Twodimensional Genus Topology: Measuring the Shape of the Matter Power Spectrum 
ARCHIVE  2004.01424 
FILE  
JOURNAL  ASTROPHYSICAL JOURNAL, 2020 
ABSTRACT  We present measurements of the twodimensional genus of the SDSSIII Baryon Oscillation Spectroscopic Survey (BOSS) catalogs to constrain cosmological parameters governing the shape of the matter power spectrum. The BOSS data are divided into 12 concentric shells over the redshift range 0.2 z < 0.6, and we extract the genus from the projected twodimensional galaxy density fields. We compare the genus amplitudes to their Gaussian expectation values, exploiting the fact that this quantity is relatively insensitive to nonlinear gravitational collapse. The genus amplitude provides a measure of the shape of the linear matter power spectrum and is principally sensitive to Omega(c)h(2) and scalar spectral index n(s). A strong negative degeneracy between Omega(c)h(2) and n(s) is observed, as both can increase smallscale power by shifting the peak and tilting the power spectrum, respectively. We place a constraint on the particular combination n(s)(3/2) Omega(c)h(2)we find n(s)(3/2) Omega(c)h(2) = 0.1121 +/ 0.0043 after combining the LOWZ and CMASS data sets, assuming a flat ?CDM cosmology. This result is practically insensitive to reasonable variations of the power spectrum amplitude and linear galaxy bias. Our results are consistent with the Planck best fit n(s)(3/2) Omega(c)h(2) = 0.1139 +/ 0.0009. 

Publication at kias
NUMBER  
AUTHOR  Kim, Juhan 
TITLE  HIR4: cosmology from a simulated neutral hydrogen full sky using Horizon Run 4 
ARCHIVE  
FILE  
JOURNAL  MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 2020 
ABSTRACT  The distribution of cosmological neutral hydrogen will provide a new window into the largescale structure of the Universe with the next generation of radio telescopes and surveys. The observation of this material, through 21 cm line emission, will be confused by foreground emission in the same frequencies. Even after these foregrounds are removed, the reconstructed map may not exactly match the original cosmological signal, which will introduce systematic errors and offset into the measured correlations. In this paper, we simulate future surveys of neutral hydrogen using the Horizon Run 4 (HR4) cosmological Nbody simulation. We generate HI intensity maps from the HR4 halo catalogue, and combine with foreground radio emission maps from the Global Sky Model, to create accurate simulations over the entire sky. We simulate the HI sky for the frequency range 700800 MHz, matching the sensitivity of the Tianlai pathfinder. We test the accuracy of the fastICA, PCA, and logpolynomial fitting foreground removal methods to recover the input cosmological angular power spectrum and measure the parameters. We show the effect of survey noise levels and beam sizes on the recovered the cosmological constraints. We find that while the reconstruction removes power from the cosmological 21 cm distribution on large scales, we can correct for this and recover the input parameters in the noisefree case. However, the effect of noise and beam size of the Tianlai pathfinder prevents accurate recovery of the cosmological parameters when using only intensity mapping information. 

Publication at kias
NUMBER  K21005 
AUTHOR  Park, Changbom,Hwang, Ho Seong,Hong, Sungryong,Kim, Juhan 
TITLE  Constraining cosmology with big data statistics of cosmological graphs 
ARCHIVE  arXiv:1903.07626 
FILE  
JOURNAL  MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 2020 
ABSTRACT  By utilizing largescale graph analytic tools implemented in the modern big data platform, APACHE SPARK, we investigate the topological structure of gravitational clustering in five different universes produced by cosmological Nbody simulations with varying parameters: (1) a WMAP 5yr compatible Lambda CDM cosmology, (2) two different dark energy equation of state variants, and (3) two different cosmic matter density variants. For the big data calculations, we use a custom build of standalone Spark/Hadoop cluster at Korea Institute for Advanced Study and Dataproc Compute Engine in Google Cloud Platform with sample sizes ranging from 7 to 200 million. We find that among the many possible graphtopological measures, three simple ones: (1) the average of number of neighbours (the socalled average vertex degree) alpha, (2) closedtoconnected triple fraction (the socalled transitivity) tau(Delta), and (3) the cumulative number density n(s) (>= 5) of subgraphs with connected component size s >= 5, can effectively discriminate among the five model universes. Since these graphtopological measures are directly related with the usual npoints correlation functions of the cosmic density field, graphtopological statistics powered by big data computational infrastructure opens a new, intuitive, and computationally efficient window into the dark Universe. 

Publication at kias
NUMBER  K21003 
AUTHOR  Moon, JunSung,Kim, Juhan 
TITLE  Living with Neighbors. II. Statistical Analysis of Flybys and Mergers of Dark Matter Halos in Cosmological Simulations 
ARCHIVE  arXiv:1911.11782 
FILE  
JOURNAL  ASTROPHYSICAL JOURNAL, 2019 
ABSTRACT  We present a statistical analysis of the flybys of dark matter halos compared to mergers, using cosmological Nbody simulations. We mainly focus on gravitationally interacting target halos with mass of 10(10.8)10(13.0) h(1) Mcircle dot, and their neighbors are counted only when the mass ratio is 1:33:1 and the distance is less than the sum of the virial radii of target and neighbor. The neighbors are divided into the flyby or merger samples if the pairs total energy is greater or smaller, respectively, than the capture criterion with consideration of dynamical friction. The main results are as follows: (a) the flyby fraction increases by up to a factor of 50 with decreasing halo mass and by up to a factor of 400 with increasing largescale density, while the merger fraction does not show any significant dependencies on these two parameters; (b) the redshift evolution of the flyby fraction is twofold, increasing with redshift at 0 < z < 1 and remaining constant at z > 1, while the merger fraction increases monotonically with redshift at z = 0 similar to 4; (c) Multiple interactions with two or more neighbors are on average flybydominated, and their fraction has a mass and environment dependence similar to that for the flyby fraction; and (d) Given that flybys substantially outnumber mergers toward z = 0 (by a factor of five) and the multiple interactions are flybydominated, the flybys contribution to galactic evolution is stronger than ever at the present epoch, especially for less massive halos and in the higher density environment. We propose a scenario that connects the evolution of the flyby and merger fractions to the hierarchical structure formation process. 

Publication at kias
NUMBER  K21004 
AUTHOR  Kim, Juhan 
TITLE  Efficient Parallel Algorithm for Estimating Higherorder Polyspectra 
ARCHIVE  arXiv:1904.11055 
FILE  
JOURNAL  ASTRONOMICAL JOURNAL, 2019 
ABSTRACT  Nonlinearities in the gravitational evolution, galaxy bias, and redshiftspace distortion drive the observed galaxy density fields away from the initial nearGaussian states. Exploiting such a nonGaussian galaxy density field requires measuring higherorder correlation functions, or, its Fourier counterpart, polyspectra. Here, we present an efficient parallel algorithm for estimating higherorder polyspectra. Based upon the Scoccimarro estimator, the estimator avoids direct sampling of polygons using the fast Fourier transform, and the parallelization overcomes the large memory requirement of the original estimator. In particular, we design the memory layout to minimize the interCPU communications, which excels in the code performance. 

Publication at kias
NUMBER  P19037 
AUTHOR  Park, Changbom,Zheng, Yi,Kim, Juhan,Park, Hyunbae,Tonegawa, Motonari 
TITLE  AlcockPaczynski Test with the Evolution of Redshiftspace Galaxy Clustering Anisotropy 
ARCHIVE  arXiv:1904.05503 
FILE  
JOURNAL  ASTROPHYSICAL JOURNAL, 2019 
ABSTRACT  We develop an improved AlcockPaczynski (AP) test method that uses the redshiftspace twopoint correlation function (2pCF) of galaxies. Cosmological constraints can be obtained by examining the redshift dependence of the normalized 2pCF, which should not change apart from the expected small nonlinear evolution. An incorrect choice of cosmology used to convert redshift to comoving distance will manifest itself as redshiftdependent 2pCF. Our method decomposes the redshift difference of the twodimensional correlation function into the Legendre polynomials whose amplitudes are modeled by radial fitting functions. Our likelihood analysis with this 2D fitting scheme tightens the constraints on Omega(m) and w by similar to 40% compared to the method of Li et al. that uses onedimensional angular dependence only. We also find that the correction for the nonlinear evolution in the 2pCF has a nonnegligible cosmology dependence, which has been neglected in previous similar studies by Li et al. With an accurate accounting for the nonlinear systematics and use of full twodimensional shape information of the 2pCF down to scales as small as 5 h(1) Mpc it is expected that the AP test with redshiftspace galaxy clustering anisotropy can be a powerful method to constraining the expansion history of the universe. 

Publication at kias
NUMBER  
AUTHOR  Kim, Juhan 
TITLE  Graph Database Solution for Higherorder Spatial Statistics in the Era of Big Data 
ARCHIVE  
FILE  
JOURNAL  ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 2019 
ABSTRACT  We present an algorithm for the fast computation of the general Npoint spatial correlation functions of any discrete point set embedded within an Euclidean space of Rn. Utilizing the concepts of kdtrees and graph databases, we describe how to count all possible Ntuples in binned configurations within a given length scale, e.g., all pairs of points or all triplets of points with side lengths < r(MAX). Through benchmarking, we show the computational advantage of our new graphbased algorithm over more traditional methods. We show measurements of the threepoint correlation function up to scales of similar to 200 Mpc (beyond the baryon acoustic oscillation scale in physical units) using current Sloan Digital Sky Survey (SDSS) data. Finally, we present a preliminary exploration of the smallscale fourpoint correlation function of 568,776 SDSS Constant (stellar) Mass (CMASS) galaxies in the northern Galactic cap over the redshift range of 0.43 < z < 0.7. We present the publicly available code GRAMSCI (GRAph Made Statistics for Cosmological Information; bitbucket.org/csabiu /gramsci), under a Gnu is Not Unix (GNU) General Public License. 

Publication at kias
NUMBER  
AUTHOR  Park, Changbom,Kim, Juhan 
TITLE  Supercluster A2142 and collapse in action: infalling and merging groups and galaxy transformations 
ARCHIVE  
FILE  
JOURNAL  ASTRONOMY & ASTROPHYSICS, 2018 
ABSTRACT  Context. Superclusters with collapsing cores represent dynamically evolving environments for galaxies, galaxy groups, and clusters. Aims. We study the dynamical state and properties of galaxies and groups in the supercluster SC1 A2142 that has a collapsing core, to understand its possible formation and evolution. Methods. We find the substructure of galaxy groups using normal mixture modelling. We have used the projected phase space (PPS) diagram, spherical collapse model, clustercentric distances, and magnitude gap between the brightest galaxies in groups to study the dynamical state of groups and to analyse group and galaxy properties. We compared the alignments of groups and their brightest galaxies with the supercluster axis. Results. The supercluster core has a radius of about 8 h(1) Mpc and total mass Mtot approximate to 2.3 x 10(15) h(1) Mcircle dot and is collapsing. Galaxies in groups on the supercluster axis have older stellar populations than offaxis groups, with median stellar ages 46 and <4 Gyr, correspondingly. The cluster A2142 and the group Gr8 both host galaxies with the oldest stellar populations among groups in SC1 A2142 having the median stellar age t > 8 Gyr. Recently quenched galaxies and active galactic nuclei (AGNs) are mostly located at virial radii or in merging regions of groups, and at clustercentric distances Dc approximate to 6 h(1) Mpc. The most elongated groups lie along the supercluster axis and are aligned with it. Magnitude gaps between the brightest galaxies of groups are less than one magnitude, suggesting that groups in SC1 A2142 are dynamically young. Conclusions. The collapsing core of the supercluster, infall of galaxies and groups, and possible merging groups, which affect galaxy properties and may trigger the activity of AGNs, show how the whole supercluster is evolving. 

Publication at kias
NUMBER  P18069 
AUTHOR  Park, Changbom ,Kim, Juhan,Snaith, Owain Nicholas 
TITLE  Resolution Convergence in Cosmological Hydrodynamical Simulations Using Adaptive Mesh Refinement 
ARCHIVE  astroph/1803.08061 
FILE  
JOURNAL  Monthly Notices of the Royal Astronomical Society,, 2018 
ABSTRACT  We have explored the evolution of gas distributions from cosmological simulations carried out using the RAMSES adaptive mesh refinement (AMR) code, to explore the effects of resolution on cosmological hydrodynamical simulations. It is vital to understand the effect of both the resolution of initial conditions (ICs) and the final resolution of the simulation. Lower initial resolution simulations tend to produce smaller numbers of lowmass structures. This will strongly affect the assembly history of objects, and has the same effect of simulating different cosmologies. The resolution of ICs is an important factor in simulations, even with a fixed maximum spatial resolution. The power spectrum of gas in simulations using AMR diverges strongly from the fixed grid approach  with more power on small scales in the AMR simulations  even at fixed physical resolution and also produces offsets in the star formation at specific epochs. This is because before certain times the upper grid levels are held back to maintain approximately fixed physical resolution, and to mimic the natural evolution of dark matter only simulations. Although the impact of holdback falls with increasing spatial and IC resolutions, the offsets in the star formation remain down to a spatial resolution of 1 kpc. These offsets are of the order of 1020 per cent, which is below the uncertainty in the implemented physics but are expected to affect the detailed properties of galaxies. We have implemented a new gridholdback approach to minimize the impact of holdback on the star formation rate. 

Publication at kias
NUMBER  K17036 
AUTHOR  Park, Changbom,Kim, Juhan,Pichon, Christophe 
TITLE  Cylinders out of a top hat: countsincells for projected densities 
ARCHIVE  arXiv:1711.04767 
FILE  
JOURNAL  MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 2018 
ABSTRACT  Large deviation statistics is implemented to predict the statistics of cosmic densities in cylinders applicable to photometric surveys. It yields few per cent accurate analytical predictions for the onepoint probability distribution function (PDF) of densities in concentric or compensated cylinders; and also captures the density dependence of their angular clustering (cylinder bias). All predictions are found to be in excellent agreement with the cosmological simulation Horizon Run 4 in the quasilinear regime where standard perturbation theory normally breaks down. These results are combined with a simple local bias model that relates dark matter and tracer densities in cylinders and validated on simulated halo catalogues. This formalism can be used to probe cosmology with existing and upcoming photometric surveys like DES, Euclid or WFIRST containing billions of galaxies. 

Publication at kias
NUMBER  P18041 
AUTHOR  Park, Changbom,Kim, Juhan,Snaith, Owain N. 
TITLE  Resolution convergence in cosmological hydrodynamical simulations using adaptive mesh refinement 
ARCHIVE  
FILE  
JOURNAL  MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 2018 
ABSTRACT  We have explored the evolution of gas distributions from cosmological simulations carried out using the RAMSES adaptive mesh refinement (AMR) code, to explore the effects of resolution on cosmological hydrodynamical simulations. It is vital to understand the effect of both the resolution of initial conditions (ICs) and the final resolution of the simulation. Lower initial resolution simulations tend to produce smaller numbers of lowmass structures. This will strongly affect the assembly history of objects, and has the same effect of simulating different cosmologies. The resolution of ICs is an important factor in simulations, even with a fixed maximum spatial resolution. The power spectrum of gas in simulations using AMR diverges strongly from the fixed grid approach with more power on small scales in the AMR simulations even at fixed physical resolution and also produces offsets in the star formation at specific epochs. This is because before certain times the upper grid levels are held back to maintain approximately fixed physical resolution, and to mimic the natural evolution of dark matter only simulations. Although the impact of holdback falls with increasing spatial and IC resolutions, the offsets in the star formation remain down to a spatial resolution of 1 kpc. These offsets are of the order of 1020 per cent, which is below the uncertainty in the implemented physics but are expected to affect the detailed properties of galaxies. We have implemented a new gridholdback approach to minimize the impact of holdback on the star formation rate. 

Publication at kias
NUMBER  P18057 
AUTHOR  Park, Changbom,Kim, Juhan,Appleby, Stephen,Appleby, Stephen 
TITLE  Minkowski Tensors in Two Dimensions: Probing the Morphology and Isotropy of the Matter and Galaxy Density Fields 
ARCHIVE  1712.07466 
FILE  
JOURNAL  ASTROPHYSICAL JOURNAL, 2018 
ABSTRACT  We apply the Minkowski tensor statistics to twodimensional slices of the threedimensional matter density field. The Minkowski tensors are a set of functions that are sensitive to directionally dependent signals in the data and, furthermore, can be used to quantify the mean shape of density fields. We begin by reviewing the definition of Minkowski tensors and introducing a method of calculating them from a discretely sampled field. Focusing on the statistic W2(1,1)a 2 x 2 matrixwe calculate its value for both the entire excursion set and individual connected regions and holes within the set. To study the morphology of structures within the excursion set, we calculate the eigenvalues lambda(1), lambda(2) for the matrix W2(1,1) of each distinct connected region and hole and measure their mean shape using the ratio beta equivalent to . We compare both W2(1,1) and beta for a Gaussian field and a smoothed density field generated from the latest Horizon Run 4 cosmological simulation to study the effect of gravitational collapse on these functions. The global statistic W2(1,1) is essentially independent of gravitational collapse, as the process maintains statistical isotropy. However, beta is modified significantly, with overdensities becoming relatively more circular compared to underdensities at low redshifts. When applying the statistics to a redshiftspace distorted density field, the matrix W2(1,1) is no longer proportional to the identity matrix, and measurements of its diagonal elements can be used to probe the largescale velocity field. 

Publication at kias
NUMBER  
AUTHOR  Park, Changbom,Kim, Juhan,Li, XiaoDong 
TITLE  Cosmological Constraints from the Redshift Dependence of the AlcockPaczynski Effect: Dynamical Dark Energy 
ARCHIVE  
FILE  
JOURNAL  ASTROPHYSICAL JOURNAL, 2018 
ABSTRACT  We perform an anisotropic clustering analysis of 1,133,326 galaxies from the Sloan Digital Sky Survey (SDSSIII) Baryon Oscillation Spectroscopic Survey Data Release 12 covering the redshift range 0.15 < z < 0.69. The geometrical distortions of the galaxy positions, caused by incorrect assumptions in the cosmological model, are captured in the anisotropic twopoint correlation function on scales of 640 h(1) Mpc. The redshift evolution of this anisotropic clustering is used to place constraints on the cosmological parameters. We improve the methodology of Li et al. to enable efficient exploration of highdimensional cosmological parameter spaces, and apply it to the ChevallierPolarskiLinder parameterization of dark energy, w = w(0) + w(a)z/(1 + z). In combination with data on the cosmic microwave background, baryon acoustic oscillations, Type Ia supernovae, and H0 from Cepheids, we obtain Omega(m) = 0.301 +/ 0.008, w(0) = 1.042 +/ 0.067, and w(a) = 0.07 +/ 0.29 (68.3% CL). Adding our new AlcockPaczynski measurements to the aforementioned results reduces the error bars by similar to 30%40% and improves the darkenergy figure of merit by a factor of similar to 2. We check the robustness of the results using realistic mock galaxy catalogs. 

Publication at kias
NUMBER  K17029 
AUTHOR  Park, C,Kim, J,Shin, J,Pichon, C 
TITLE  A question of separation: disentangling tracer bias and gravitational nonlinearity with countsincells statistics 
ARCHIVE  arXiv1705.08901 
FILE  
JOURNAL  MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 2018 
ABSTRACT  Starting from a very accurate model for densityincells statistics of dark matter based on large deviation theory, a bias model for the tracer density in spheres is formulated. It adopts a mean bias relation based on a quadratic bias model to relate the logdensities of dark matter to those of massweighted dark haloes in real and redshift space. The validity of the parametrized bias model is established using a parametrizationindependent extraction of the bias function. This average bias model is then combined with the dark matter PDF, neglecting any scatter around it: it nevertheless yields an excellent model for densitiesincells statistics of mass tracers that is parametrized in terms of the underlying dark matter variance and three bias parameters. The procedure is validated on measurements of both the oneand twopoint statistics of subhalo densities in the stateoftheart Horizon Run 4 simulation showing excellent agreement for measured dark matter variance and bias parameters. Finally, it is demonstrated that this formalism allows for a joint estimation of the nonlinear dark matter variance and the bias parameters using solely the statistics of subhaloes. Having verified that galaxy counts in hydrodynamical simulations sampled on a scale of 10 Mpc h(1) closely resemble those of subhaloes, this work provides important steps towards making theoretical predictions for densityincells statistics applicable to upcoming galaxy surveys like Euclid or WFIRST. 

Publication at kias
NUMBER  P18058 
AUTHOR  Park, Changbom,Kim, Juhan,Appleby, Stephen,Appleby, Stephen 
TITLE  Cosmological Parameter Estimation Using the Genus AmplitudeApplication to Mock Galaxy Catalogs 
ARCHIVE  1801.06342 
FILE  
JOURNAL  ASTROPHYSICAL JOURNAL, 2018 
ABSTRACT  We study the topology of the matter density field in twodimensional slices and consider how we can use the amplitude A of the genus for cosmological parameter estimation. Using the latest Horizon Run 4 simulation data, we calculate the genus of the smoothed density field constructed from light cone mock galaxy catalogs. Information can be extracted from the amplitude of the genus by considering both its redshift evolution and magnitude. The constancy of the genus amplitude with redshift can be used as a standard population, from which we derive constraints on the equation of state of dark energy w(de)by measuring A at z similar to 0.1 and z similar to 1, we can place an order Delta w(de) similar to O(15%) constraint on w(de). By comparing A to its Gaussian expectation value, we can potentially derive an additional stringent constraint on the matter density Delta Omega(mat) similar to 0.01. We discuss the primary sources of contamination associated with the two measurementsredshift space distortion (RSD) and shot noise. With accurate knowledge of galaxy bias, we can successfully remove the effect of RSD, and the combined effect of shot noise and nonlinear gravitational evolution is suppressed by smoothing over suitably large scales RG >= 15 Mpc/h. Without knowledge of the bias, we discuss how joint measurements of the twoand threedimensional genus can be used to constrain the growth factor beta = f/b. The method can be applied optimally to redshift slices of a galaxy distribution generated using the dropoff technique. 

Publication at kias
NUMBER  K17030 
AUTHOR  Park, Changbom ,Kim, Juhan 
TITLE  Postreionization Kinetic SunyaevZel¡¯dovich Effect in Illustris Simulation 
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JOURNAL  ÇÑ±¹Ãµ¹®ÇÐÈ¸º¸, 2017 
ABSTRACT  

Publication at kias
NUMBER  K17003 
AUTHOR  Hong, Sungwook E.,Kim, Juhan,Li, XiaoDong,Park, Changbom 
TITLE  Cosmological Constraints from the Redshift Dependence of the Volume Effect Using the Galaxy 2point Correlation Function across the Line of Sight 
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JOURNAL  ASTROPHYSICAL JOURNAL, 2017 
ABSTRACT  We develop a methodology to use the redshift dependence of the galaxy 2point correlation function (2pCF) across the line of sight, xi(r(perpendicular to)), as a probe of cosmological parameters. The positions of galaxies in comoving Cartesian space varies under different cosmological parameter choices, inducing a redshiftdependent scaling in the galaxy distribution. This geometrical distortion can be observed as a redshiftdependent rescaling in the measured xi(r(perpendicular to)). We test this methodology using a sample of 1.75 billion mock galaxies at redshifts 0, 0.5, 1, 1.5, and 2, drawn from the Horizon Run 4 Nbody simulation. The shape of xi(r(perpendicular to)) can exhibit a significant redshift evolution when the galaxy sample is analyzed under a cosmology differing from the true, simulated one. Other contributions, including the gravitational growth of structure, galaxy bias, and the redshift space distortions, do not produce large redshift evolution in the shape. We show that one can make use of this geometrical distortion to constrain the values of cosmological parameters governing the expansion history of the universe. This method could be applicable to future largescale structure surveys, especially photometric surveys such as DES and LSST, to derive tight cosmological constraints. This work is a continuation of our previous works as a strategy to constrain cosmological parameters using redshiftinvariant physical quantities. 

Publication at kias
NUMBER  K17002 
AUTHOR  Park, Changbom,Kim, Juhan,Shin, Jihye,Pichon, Christophe 
TITLE  New Fitting Formula for Cosmic Nonlinear Density Distribution 
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JOURNAL  ASTROPHYSICAL JOURNAL, 2017 
ABSTRACT  We have measured the probability distribution function (PDF) of a cosmic matter density field from a suite of Nbody simulations. We propose the generalized normal distribution of version 2 (Nv2) as an alternative fitting formula to the wellknown lognormal distribution. We find that Nv2 provides a significantly better fit than that of the lognormal distribution for all smoothing radii (2, 5, 10, 25 [Mpc h(1)]) that we studied. The improvement is substantial in the underdense regions. The development of nonGaussianities in the cosmic matter density field is captured by continuous evolution of the skewness and shift parameters of the Nv2 distribution. We present the redshift evolution of these parameters for aforementioned smoothing radii and various background cosmology models. All the PDFs measured from large and highresolution Nbody simulations that we use in this study can be obtained from the web site https://astro.kias.re.kr/jhshin. 

Publication at kias
NUMBER  K17004 
AUTHOR  LHuillier, Benjamin,Kim, Juhan,Park, Changbom 
TITLE  Dark matter haloes in modified gravity and dark energy: interaction rate, small and largescale alignment 
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JOURNAL  MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 2017 
ABSTRACT  We study the properties of dark matter haloes in a wide range of modified gravity models, namely, f(R), DGP and interacting dark energy models. We study the effects of modified gravity and dark energy on the internal properties of haloes, such as the spin and the structural parameters. We find that f(R) gravity enhances the median value of the Bullock spin parameter, but could not detect such effects for DGP and coupled dark energy. f(R) also yields a lower median sphericity and oblateness, while coupled dark energy has the opposite effect. However, these effects are very small. We then study the interaction rate of haloes in different gravity and find that only strongly coupled dark energy models enhance the interaction rate. We then quantify the enhancement of the alignment of the spins of interacting halo pairs by modified gravity. Finally, we study the alignment of the major axes of haloes with the largescale structures. The alignment of the spins of interacting pairs of haloes in DGP and coupled dark energy models show no discrepancy with GR, while f(R) shows a weaker alignment. Strongly coupled dark energy shows a stronger alignment of the halo shape with the largescale structures. 

Publication at kias
NUMBER  K17005 
AUTHOR  LHuillier, Benjamin,Kim, Juhan,Park, Changbom 
TITLE  Ecology of dark matter haloes  II. Effects of interactions on the alignment of halo pairs 
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JOURNAL  MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 2017 
ABSTRACT  We use the Horizon Run 4 cosmological Nbody simulation to study the effects of distant and close interactions on the alignments of the shapes, spins and orbits of targets haloes with their neighbours, and their dependence on the local density environment and neighbour separation. Interacting targets have a significantly lower spin and higher sphericity and oblateness than all targets. Interacting pairs initially have antiparallel spins, but the spins develop parallel alignment as time goes on. Neighbours tend to evolve in the plane of rotation of the target, and in the direction of the major axis of prolate haloes. Moreover, interactions are preferentially radial, while pairs with nonradial orbits are preferentially prograde. The alignment signals are stronger at high mass and for close separations, and independent of the largescale density. Positive alignment signals are found at redshifts up to 4, and increase with decreasing redshifts. Moreover, the orbits tend to become prograde at low redshift, while no alignment is found at high redshift (z = 4). 

Publication at kias
NUMBER  K17037 
AUTHOR  LHuillier, B.,Kim, J.,Pichon, C.,Park, C. 
TITLE  Beyond Kaiser bias: mildly nonlinear twopoint statistics of densities in distant spheres 
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JOURNAL  MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 2017 
ABSTRACT  We present simple parameterfree analytic bias functions for the twopoint correlation of densities in spheres at large separation. These bias functions generalize the socalled Kaiser bias to the mildly nonlinear regime for arbitrary density contrasts and grow as b(rho) b(1) proportional to (1  rho(13/21))rho(1 + n/3) with b(1) = 4/21  n/3 for a powerlaw initial spectrum with index n. We carry out the derivation in the context of largedeviation statistics while relying on the spherical collapse model. We use a logarithmic transformation that provides a saddlepoint approximation that is valid for the whole range of densities and showits accuracy against the 30 Gpc cube stateoftheart Horizon Run 4 simulation. Special configurations of two concentric spheres that allow us to identify peaks are employed to obtain the conditional bias and a proxy for the BBKS extremum correlation functions. These analytic bias functions should be used jointly with extended perturbation theory to predict twopoint clustering statistics as they capture the nonlinear regime of structure formation at the per cent level down to scales of about 10 Mpc h(1) at redshift 0. Conversely, the joint statistics also provide us with optimal dark matter twopoint correlation estimates that can be applied either universally to all spheres or to a restricted set of biased (overor underdense) pairs. Based on a simple fiducial survey, we show that the variance of this estimator is reduced by five times relative to the traditional sample estimator for the twopoint function. Extracting more information from correlations of different types of objects should prove essential in the context of upcoming surveys like Euclid, DESI and WFIRST. 

Publication at kias
NUMBER  K17006 
AUTHOR  Hong, Sungwook E.,Kim, Juhan,Appleby, Stephen,Appleby, Stephen,Park, Changbom 
TITLE  Topology of Largescale Structures of Galaxies in Two DimensionsSystematic Effects 
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JOURNAL  ASTROPHYSICAL JOURNAL, 2017 
ABSTRACT  We study the twodimensional topology of galactic distribution when projected onto twodimensional spherical shells. Using the latest Horizon Run 4 simulation data, we construct the genus of the twodimensional field and consider how this statistic is affected by latetime nonlinear effectsprincipally gravitational collapse and redshift space distortion (RSD). We also consider systematic and numerical artifacts, such as shot noise, galaxy bias, and finite pixel effects. We model the systematics using a Hermite polynomial expansion and perform a comprehensive analysis of known effects on the twodimensional genus, with a view toward using the statistic for cosmological parameter estimation. We find that the finite pixel effect is dominated by an amplitude drop and can be made less than 1% by adopting pixels smaller than 1/3 of the angular smoothing length. Nonlinear gravitational evolution introduces timedependent coefficients of the zeroth, first, and second Hermite polynomials, but the genus amplitude changes by less than 1% between z = 1 and z = 0 for smoothing scales RG > 9 Mpc/h. Nonzero terms are measured up to third order in the Hermite polynomial expansion when studying RSD. Differences in the shapes of the genus curves in real and redshift space are small when we adopt thick redshift shells, but the amplitude change remains a significant similar to O(10%) effect. The combined effects of galaxy biasing and shot noise produce systematic effects up to the second Hermite polynomial. It is shown that, when sampling, the use of galaxy mass cuts significantly reduces the effect of shot noise relative to random sampling. 

Publication at kias
NUMBER  K17007 
AUTHOR  Hong, Sungwook E.,Kim, Juhan,Li, XiaoDong,Park, Changbom 
TITLE  COSMOLOGICAL CONSTRAINTS FROM THE REDSHIFT DEPENDENCE OF THE ALCOCKPACZYNSKI EFFECT: APPLICATION TO THE SDSSIII BOSS DR12 GALAXIES 
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JOURNAL  ASTROPHYSICAL JOURNAL, 2016 
ABSTRACT  We apply the methodology developed in Li et al. to BOSS DR12 galaxies and derive cosmological constraints from the redshift dependence of the AlcockPaczynski (AP) effect. The apparent anisotropy in the distribution of observed galaxies arise from two main sources, the redshiftspace distortion (RSD) effect due to the galaxy peculiar velocities, and the geometric distortion when incorrect cosmological models are assumed for transforming redshift to comoving distance, known as the AP effect. Anisotropies produced by the RSD effect are, although large, maintaining a nearly uniform magnitude over a large range of redshift, while the degree of anisotropies from the AP effect varies with redshift by a much larger magnitude. We split the DR12 galaxies into six redshift bins, measure the twopoint correlation function in each bin, and assess the redshift evolution of anisotropies. We obtain constraints of Omega(m) = 0.290 +/ 0.053, w =  1.07 +/ 0.15, which are comparable with the current constraints from other cosmological probes such as SNe Ia, cosmic microwave background, and baryon acoustic oscillation (BAO). Combining these cosmological probes with our method yield tight constraints of Omega(m) =0.301 +/ 0.006, w =  1.054 +/ 0.025. Our method is complementary to the other largescale structure (LSS) probes like BAO and topology. We expect this technique will play an important role in deriving cosmological constraints from LSS surveys. 

Publication at kias
NUMBER  K17001 
AUTHOR  Park, Changbom,Kim, Juhan,Hong, Sungwook E. 
TITLE  THE MOST BOUND HALO PARTICLEGALAXY CORRESPONDENCE MODEL: COMPARISON BETWEEN MODELS WITH DIFFERENT MERGER TIMESCALES 
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JOURNAL  ASTROPHYSICAL JOURNAL, 2016 
ABSTRACT  We develop a galaxy assignment scheme that populates dark matter halos with galaxies by tracing the most bound member particles (MBPs) of simulated halos. Several merger timescale models based on analytic calculations and numerical simulations are adopted as the survival times of mock satellite galaxies. We build mock galaxy samples from halo merger data of the Horizon Run 4 Nbody simulation from z = 120. We compare group properties and twopoint correlation functions (2pCFs) of mock galaxies with those of volumelimited SDSS galaxies, with rband absolute magnitudes of Mr 5 log h < 21 and 20 at z = 0. It is found that the MBPgalaxy correspondence scheme reproduces the observed population of SDSS galaxies in massive galaxy groups (M > 10(14) h(1) Mcircle dot) and the smallscale 2pCF (r(p) < 10 h(1) Mpc) quite well for the majority of the merger timescale models adopted. The new scheme outperforms the previous subhalogalaxy correspondence scheme by more than 2 sigma. 

Publication at kias
NUMBER  Q16007 
AUTHOR  Park, Changbom,Hwang, Ho Seong,Kim, Juhan 
TITLE  HECTOMAP AND HORIZON RUN 4: DENSE STRUCTURES AND VOIDS IN THE REAL AND SIMULATED UNIVERSE 
ARCHIVE  arXiv:1602.06343 
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JOURNAL  ASTROPHYSICAL JOURNAL, 2016 
ABSTRACT  HectoMAP is a dense redshift survey of red galaxies covering a 53 deg(2) strip of the northern sky. HectoMAP is 97% complete for galaxies with r < 20.5, (gr) > 1.0, and (r i) > 0.5. The survey enables tests of the physical properties of largescale structure at intermediate redshift against cosmological models. We use the Horizon Run 4, one of the densest and largest cosmological simulations based on the standard. Cold Dark Matter (Lambda CDM) model, to compare the physical properties of observed largescale structures with simulated ones in a volumelimited sample covering 8 x 10(6) h(3) Mpc(3) in the redshift range 0.22 < z < 0.44. We apply the same criteria to the observations and simulations to identify overand underdense largescale features of the galaxy distribution. The richness and size distributions of observed overdense structures agree well with the simulated ones. Observations and simulations also agree for the volume and size distributions of underdense structures, voids. The properties of the largest overdense structure and the largest void in HectoMAP are well within the distributions for the largest structures drawn from 300 Horizon Run 4 mock surveys. Overall the size, richness and volume distributions of observed largescale structures in the redshift range 0.22 < z < 0.44 are remarkably consistent with predictions of the standard Lambda CDM model. 

Publication at kias
NUMBER  K17009 
AUTHOR  Kim, Juhan 
TITLE  FlyBy Encounters Between Dark Matter Halos in Cosmological Simulations 
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JOURNAL  Publications of The Korean Astronomical Society, 2015 
ABSTRACT  

Publication at kias
NUMBER  K17014 
AUTHOR  Park, Changbom ,Kim, Juhan 
TITLE  Topology of neutral hydrogen distribution with the Square Kilometre Array 
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JOURNAL  Proceedings of Advancing Astrophysics with the Square Kilometre Array, 2015 
ABSTRACT  Morphology of the complex HI gas distribution can be quantified by statistics like the Minkowski functionals, and can provide a way to statistically study the large scale structure in the HI maps both at low redshifts, and during the epoch of reionization (EoR). At low redshifts, the 21cm emission traces the underlying matter distribution. Topology of the HI gas distribution, as measured by the genus, could be used as a standard ruler. This enables the determination of distanceredshift relation and also the discrimination of various models of dark energy and of modified gravity. The topological analysis is also sensitive to certain primordial nonGaussian features. Compared with twopoint statistics, the topological statistics are more robust against the nonlinear gravitational evolution, bias, and redshiftspace distortion. The HI intensity map observation naturally avoids the sparse sampling distortion, which is an important systematic in optical galaxy survey. The large cosmic volume accessible to SKA would provide unprecedented accuracy using such a measurement... [abridged] 

Publication at kias
NUMBER  K17010 
AUTHOR  Kim, Juhan 
TITLE  STOCHASTIC MODEL OF THE SPIN DISTRIBUTION OF DARK MATTER HALOS 
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JOURNAL  ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 2015 
ABSTRACT  We employ a stochastic approach to probing the origin of the lognormal distributions of halo spin in Nbody simulations. After analyzing spin evolution in halo merging trees, it was found that a spin change can be characterized by a stochastic random walk of angular momentum. Also, spin distributions generated by random walks are fairly consistent with those directly obtained from Nbody simulations. We derived a stochastic differential equation from a widely used spin definition and measured the probability distributions of the derived angular momentum change from a massive set of halo merging trees. The roles of major merging and accretion are also statistically analyzed in evolving spin distributions. Several factors (local environment, halo mass, merging mass ratio, and redshift) are found to influence the angular momentum change. The spin distributions generated in the meanfield or void regions tend to shift slightly to a higher spin value compared with simulated spin distributions, which seems to be caused by the correlated random walks. We verified the assumption of randomness in the angular momentum change observed in the Nbody simulation and detected several degrees of correlation between walks, which may provide a clue for the discrepancies between the simulated and generated spin distributions in the voids. However, the generated spin distributions in the group and cluster regions successfully match the simulated spin distribution. We also demonstrated that the lognormality of the spin distribution is a natural consequence of the stochastic differential equation of the halo spin, which is well described by the Geometric Brownian Motion model. 

Publication at kias
NUMBER  K17011 
AUTHOR  LHuillier, Benjamin,Kim, Juhan,Hong, Sungwook E.,Park, Changbom 
TITLE  HORIZON RUN 4 SIMULATION: COUPLED EVOLUTION OF GALAXIES AND LARGESCALE STRUCTURES OF THE UNIVERSE 
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JOURNAL  JOURNAL OF THE KOREAN ASTRONOMICAL SOCIETY, 2015 
ABSTRACT  The Horizon Run 4 is a cosmological Nbody simulation designed for the study of coupled evolution between galaxies and largescale structures of the Universe, and for the test of galaxy formation models. Using 6300(3) gravitating particles in a cubic box of Lbox = 3150 h(1) Mpc, we build a dense forest of halo merger trees to trace the halo merger history with a halo mass resolution scale down to Ms = 2.7 x 10(11)h(1)M(circle dot). We build a set of particle and halo data, which can serve as testbeds for comparison of cosmological models and gravitational theories with observations. We find that the FoF halo mass function shows a substantial deviation from the universal form with tangible redshift evolution of amplitude and shape. At higher redshifts, the amplitude of the mass function is lower, and the functional form is shifted toward larger values of ln(1/sigma). We also find that the baryonic acoustic oscillation feature in the twopoint correlation function of mock galaxies becomes broader with a peak position moving to smaller scales and the peak amplitude decreasing for increasing directional cosine mu, compared to the linear predictions. From the halo merger trees built from halo data at 75 redshifts, we measure the halfmass epoch of halos and find that less massive halos tend to reach half of their current mass at higher redshifts. Simulation outputs including snapshot data, past lightcone space data, and halo merger data are available at http://sdss.kias.re.kr/astro/HorizonRun4. 

Publication at kias
NUMBER  K17015 
AUTHOR  Park, Changbom,Kim, Juhan 
TITLE  HORIZON RUN 3: TOPOLOGY AS A STANDARD RULER 
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JOURNAL  ASTROPHYSICAL JOURNAL, 2015 
ABSTRACT  We study the physically selfbound cold dark matter halo distribution, which we associate with the massive galaxies within Horizon Run 3, to estimate the accuracy of the determination of the cosmological distance scale measured by the topology analysis. We apply the routine Contour 3D to the 108 Mock Survey of p steradians out to redshift z = 0.6, which effectively corresponds to the SDSSIII Baryon Oscillation Spectroscopic Survey (BOSS) survey, and compare the topology with that of a Gaussian random phase field. We find that given three separate smoothing lengths. = 15, 21, and 34 h(1) Mpc, the least chi(2) fit genus per unit volume (g) yields a 1.7% fractional uncertainty in smoothing length and angular diameter distance to z = 0.6. This is an improvement on former calibrations and presents an error estimate competitive with baryon acoustic oscillation scale techniques. We also present threedimensional graphics of the Horizon Run 3 spherical mock survey to show a wealth of largescale structures of the universe that are expected for surveys like BOSS. 

Publication at kias
NUMBER  K17012 
AUTHOR  LHuillier, Benjamin,Kim, Juhan,Park, Changbom 
TITLE  The ecology of dark matter haloes I. The rates and types of halo interactions 
ARCHIVE  
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JOURNAL  MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 2015 
ABSTRACT  Interactions such as mergers and flybys play a fundamental role in shaping galaxy morphology. Using the Horizon Run 4 cosmological Nbody simulation, we studied the frequency and type of halo interactions, and their redshift evolution as a function of the environment defined by the largescale density, pair separation, mass ratio, and target halo mass. Most interactions happen at largescale density contrast delta similar or equal to 20, regardless of the redshift corresponding to groups and relatively dense part of filaments. However, the fraction of interacting target is maximum at delta similar or equal to 1000. We provide a new empirical fitting form for the interaction rate as a function of the halo mass, largescale density, and redshift. We also report the existence of two modes of interactions from the distributions of mass ratio and relative distance, implying two different physical origins of the interaction. Satellite targets lose their mass as they proceed deeper into the host halo. The relative importance of these two trends strongly depends on the largescale density, target mass, and redshift. 

Publication at kias
NUMBER  K17013 
AUTHOR  Sabiu, Cristiano G.,Kim, Juhan,Li, XiaoDong,Park, Changbom 
TITLE  Cosmological constraints from the redshift dependence of the AlcockPaczynski test and volume effect: galaxy twopoint correlation function 
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JOURNAL  MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 2015 
ABSTRACT  We propose a method using the redshift dependence of the AlcockPaczynski (AP) test and volume effect to measure the cosmic expansion history. The galaxy twopoint correlation function as a function of angle, xi(mu), is measured at different redshifts. Assuming an incorrect cosmological model to convert galaxy redshifts to distances, the shape of xi(mu) appears anisotropic due to the AP effect, and the amplitude is shifted by the change in comoving volume. Due to the redshift dependence of the AP and volume effect, both the shape and amplitude of xi(mu) exhibit redshift dependence. Similar to Li et al. (2014), we find that the redshiftspace distortions (RSD) caused by galaxy peculiar velocities, although significantly distorting xi(mu), exhibit much less redshift evolution compared to the AP and volume effects. By focusing on the redshift dependence of xi(mu), we can correctly recover the cosmological parameters despite the contamination of RSD. The method is tested by using the Horizon Run 3 Nbody simulation, from which we made a series of 1/8sky mock surveys having eight million physically selfbound haloes and sampled to have roughly a uniform number density in z = 01.5. We find the AP effect results in tight, unbiased constraints on the density parameter and dark energy equation of state, with 68.3% CL intervals delta Omega(m) similar to 0.03 and delta w similar to 0.1, and the volume effect leads to much tighter constraints of delta Omega(m) similar to 0.007 and delta w similar to 0.035. 

Publication at kias
NUMBER  K17016 
AUTHOR  Park, Changbom,Kim, Juhan,Li, XiaoDong 
TITLE  COSMOLOGICAL CONSTRAINTS FROM THE REDSHIFT DEPENDENCE OF THE ALCOCKPACZYNSKI TEST: GALAXY DENSITY GRADIENT FIELD 
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JOURNAL  ASTROPHYSICAL JOURNAL, 2014 
ABSTRACT  We propose a method based on the redshift dependence of the AlcockPaczynski ( AP) test to measure the expansion history of the universe. It uses the isotropy of the galaxy density gradient field to constrain cosmological parameters. If the density parameter Omega(m) or the dark energy equation of state w are incorrectly chosen, the gradient field appears to be anisotropic with the degree of anisotropy varying with redshift. We use this effect to constrain the cosmological parameters governing the expansion history of the universe. Although redshiftspace distortions ( RSD) induced by galaxy peculiar velocities also produce anisotropies in the gradient field, these effects are close to uniform in magnitude over a large range of redshift. This makes the redshift variation of the gradient field anisotropy relatively insensitive to the RSD. By testing the method on mock surveys drawn from the Horizon Run 3 cosmological Nbody simulations, we demonstrate that the cosmological parameters can be estimated without bias. Our method is complementary to the baryon acoustic oscillation or topology methods as it depends on DAH, the product of the angular diameter distance and the Hubble parameter. 

Publication at kias
NUMBER  K17017 
AUTHOR  Kim, Juhan 
TITLE  A TOPOLOGICAL ANALYSIS OF LARGESCALE STRUCTURE, STUDIED USING THE CMASS SAMPLE OF SDSSIII 
ARCHIVE  
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JOURNAL  ASTROPHYSICAL JOURNAL, 2014 
ABSTRACT  We study the threedimensional genus topology of largescale structure using the northern region of the CMASS Data Release 10 (DR10) sample of the SDSSIII Baryon Oscillation Spectroscopic Survey. We select galaxies with redshift 0.452 < z < 0.625 and with a stellar mass Mstellar > 10(11.56) Mcircle dot. We study the topology at two smoothing lengths: RG = 21 h(1) Mpc and RG = 34 h(1) Mpc. The genus topology studied at the RG = 21 h(1) Mpc scale results in the highest genus amplitude observed to date. The CMASS sample yields a genus curve that is characteristic of one produced by Gaussian random phase initial conditions. The data thus support the standard model of inflation where random quantum fluctuations in the early universe produced Gaussian random phase initial conditions. Modest deviations in the observed genus from random phase are as expected from shot noise effects and the nonlinear evolution of structure. We suggest the use of a fitting formula motivated by perturbation theory to characterize the shift and asymmetries in the observed genus curve with a single parameter. We construct 54 mock SDSS CMASS surveys along the past light cone from the Horizon Run 3 (HR3) Nbody simulations, where gravitationally bound dark matter subhalos are identified as the sites of galaxy formation. We study the genus topology of the HR3 mock surveys with the same geometry and sampling density as the observational sample and find the observed genus topology to be consistent with ACDM as simulated by the HR3 mock samples. We conclude that the topology of the largescale structure in the SDSS CMASS sample is consistent with cosmological models having primordial Gaussian density fluctuations growing in accordance with general relativity to form galaxies in massive dark matter halos. 

Publication at kias
NUMBER  K17018 
AUTHOR  LHuillier, Benjamin,Kim, Juhan,Park, Changbom 
TITLE  Effects of the initial conditions on cosmological Nbody simulations 
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JOURNAL  NEW ASTRONOMY, 2014 
ABSTRACT  Cosmology is entering an era of percent level precision due to current large observational surveys. This precision in observation is now demanding more accuracy from numerical methods and cosmological simulations. In this paper, we study the accuracy of Nbody numerical simulations and their dependence on changes in the initial conditions and in the simulation algorithms. For this purpose, we use a series of cosmological Nbody simulations with varying initial conditions. We test the influence of the initial conditions, namely the preinitial configuration (preIC), the order of the Lagrangian perturbation theory (LPT), and the initial redshift (z(ini)), on the statistics associated with the large scale structures of the universe such as the halo mass function, the density power spectrum, and the maximal extent of the large scale structures. We find that glass or grid preinitial conditions give similar results at z less than or similar to 2. However, the initial excess of power in the glass initial conditions yields a subtle difference in the power spectra and the mass function at high redshifts. The LPT order used to generate the initial conditions of the simulations is found to play a crucial role. Firstorder LPT (1LPT) simulations underestimate the number of massive haloes with respect to secondorder (2LPT) ones, typically.by 2% at 10(14) h(1) M circle dot for an initial redshift of 23, and the smallscale power with an underestimation of 6% near the Nyquist frequency for z(ini) = 23. Larger underestimations are observed for lower starting redshifts. Moreover, at higher redshifts, the highmass end of the mass function is significantly underestimated in 1LPT simulations. On the other hand, when the LPT order is fixed, the starting redshift has a systematic impact on the lowmass end of the halo mass function. Lower starting redshifts yield more lowmass haloes. Finally, we compare two Nbody codes, Gadget3 and GOTPM, and find 8% differences in the power spectrum at small scales and in the lowmass end of the halo mass function. (C) 2014 Elsevier B.V. All rights reserved. 

Publication at kias
NUMBER  K17019 
AUTHOR  Park, Changbom,Kim, Juhan 
TITLE  EUNHA: A NEW COSMOLOGICAL HYDRODYNAMIC SIMULATION CODE 
ARCHIVE  
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JOURNAL  JOURNAL OF THE KOREAN ASTRONOMICAL SOCIETY, 2014 
ABSTRACT  We develop a parallel cosmological hydrodynamic simulation code designed for the study of formation and evolution of cosmological structures. The gravitational force is calculated using the TreePM method and the hydrodynamics is implemented based on the smoothed particle hydrodynamics. The initial displacement and velocity of simulation particles are calculated according to secondorder Lagrangian perturbation theory using the power spectra of dark matter and baryonic matter. The initial background temperature is given by Recfast and the temperature fluctuations at the initial particle position are assigned according to the adiabatic model. We use a timelimiter scheme over the individual time steps to capture shockfronts and to ease the timestep tension between the shock and preshock particles. We also include the astrophysical gas processes of radiative heating/cooling, star formation, metal enrichment, and supernova feedback. We test the code in several standard cases such as onedimensional Riemann problems, KelvinHelmholtz, and Sedov blast wave instability. Star formation on the galactic disk is investigated to check whether the SchmidtKennicutt relation is properly recovered. We also study global star formation history at different simulation resolutions and compare them with observations. 

Publication at kias
NUMBER  K17020 
AUTHOR  Kim, Juhan 
TITLE  HALO SPIN PARAMETER IN COSMOLOGICAL SIMULATIONS 
ARCHIVE  
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JOURNAL  JOURNAL OF THE KOREAN ASTRONOMICAL SOCIETY, 2014 
ABSTRACT  Using a cosmological ACDM simulation, we analyze the differences between the widelyused spin parameters suggested by Peebles and Bullock. The dimensionless spin parameter lambda proposed by Peebles is theoretically welljustified but includes an annoying term, the potential energy, which cannot be directly obtained from observations and is computationally expensive to calculate in numerical simulations. The Bullocks spin parameter lambda avoids this problem assuming the isothermal density profile of a virialized halo in the Newtonian potential model. However, we find that there exists a substantial discrepancy between lambda and lambda depending on the adopted potential model (Newtonian or Plummer) to calculate the halo total energy and that their redshift evolutions differ to each other significantly. Therefore, we introduce a new spin parameter, lambda , which is simply designed to roughly recover the value of lambda but to use the same halo quantities as used in lambda. If the Plummer potential is adopted, the lambda is related to the Bullocks definition as lambda = 0.80 x (1 + z)(1/12)lambda. Hence, the new spin parameter lambda distribution becomes consistent with a lognormal distribution frequently seen for the lambda while its mean value is much closer to that of lambda. On the other hand, in case of the Newtonian potential model, we obtain the relation of lambda = (1 + z)(1/8)lambda; there is no significant difference at z = 0 as found by others but lambda becomes more overestimated than lambda or lambda at higher redshifts. We also investigate the dependence of halo spin parameters on halo mass and redshift. We clearly show that although the lambda for smallmass halos with Mh < 2 x 10(12)M(circle dot) seems redshift independent after z = 1, all the spin parameters explored, on the whole, show a stronger correlation with the increasing halo mass at higher redshifts. 

Publication at kias
NUMBER  K17021 
AUTHOR  Hong, Sungwook E.,Kim, Juhan,Park, Changbom 
TITLE  2D GENUS TOPOLOGY OF 21CM DIFFERENTIAL BRIGHTNESS TEMPERATURE DURING COSMIC REIONIZATION 
ARCHIVE  
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JOURNAL  JOURNAL OF THE KOREAN ASTRONOMICAL SOCIETY, 2014 
ABSTRACT  A novel method to characterize the topology of the earlyuniverse intergalactic medium during the epoch of cosmic reionization is presented. The 21cm radiation background from high redshift is analyzed through calculation of the 2dimensional (2D) genus. The radiative transfer of hydrogenionizing photons and ionizationrate equations are calculated in a suite of numerical simulations under various input parameters. The 2D genus is calculated from the mock 21cm images of highredshift universe. We construct the 2D genus curve by varying the threshold differential brightness temperature, and compare this to the 2D genus curve of the underlying density field. We find that (1) the 2D genus curve reflects the evolutionary track of cosmic reionization and (2) the 2D genus curve can discriminate between certain reionization scenarios and thus indirectly probe the properties of radiationsources. Choosing the right beam shape of a radio antenna is found crucial for this analysis. Square Kilometre Array (SKA) is found to be a suitable apparatus for this analysis in terms of sensitivity, even though some deterioration of the data for this purpose is unavoidable under the planned size of the antenna core. 

Publication at kias
NUMBER  K17022 
AUTHOR  Kim, Juhan 
TITLE  TOPOLOGY OF LUMINOUS RED GALAXIES FROM THE SLOAN DIGITAL SKY SURVEY 
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JOURNAL  ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 2013 
ABSTRACT  We present measurements of the genus topology of luminous red galaxies (LRGs) from the Sloan Digital Sky Survey (SDSS) Data Release 7 catalog, with unprecedented statistical significance. To estimate the uncertainties in the measured genus, we construct 81 mock SDSS LRG surveys along the past light cone from Horizon Run 3, one of the largest Nbody simulations to date, which evolved 7210(3) particles in a 10,815 h(1) Mpc box. After carefully modeling and removing all known systematic effects due to finite pixel size, survey boundary, radial and angular selection functions, shot noise, and galaxy biasing, we find that the observed genus amplitude reaches 272 at a 22 h(1) Mpc smoothing scale, with an uncertainty of 4.2%; the estimated error fully incorporates cosmic variance. This is the most accurate constraint on the genus amplitude to date and significantly improves on our previous results. In particular, the shape of the genus curve agrees very well with the mean topology of the SDSS LRG mock surveys in a. cold dark matter universe. However, comparison with simulations also shows small deviations of the observed genus curve from the theoretical expectation for Gaussian initial conditions. While these discrepancies are mainly driven by known systematic effects such as shot noise and redshiftspace distortions, they do contain important cosmological information on the physical effects connected with galaxy formation, gravitational evolution, and primordial nonGaussianity. We address the key role played by systematics on the genus curve and show how to accurately correct for their effects to recover the topology of the underlying matter. A future work will provide an interpretation of these deviations in the context of the local model of nonGaussianity. 

Publication at kias
NUMBER  K17023 
AUTHOR  Kim, Juhan 
TITLE  INITIAL SIZE DISTRIBUTION OF THE GALACTIC GLOBULAR CLUSTER SYSTEM 
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JOURNAL  ASTROPHYSICAL JOURNAL, 2013 
ABSTRACT  Despite the importance of their size evolution in understanding the dynamical evolution of globular clusters (GCs) of the Milky Way, studies that focus specifically on this issue are rare. Based on the advanced, realistic FokkerPlanck (FP) approach, we theoretically predict the initial size distribution (SD) of the Galactic GCs along with their initial mass function and radial distribution. Over one thousand FP calculations in a wide parameter space have pinpointed the bestfit initial conditions for the SD, mass function, and radial distribution. Our bestfit model shows that the initial SD of the Galactic GCs is of larger dispersion than todays SD, and that the typical projected halflight radius of the initial GCs is similar to 4.6 pc, which is 1.8 times larger than that of the presentday GCs (similar to 2.5 pc). Their large size signifies greater susceptibility to the Galactic tides: the total mass of destroyed GCs reaches 35 x 10(8) Mcircle dot, several times larger than previous estimates. Our result challenges a recent view that the Milky Way GCs were born compact on the subpc scale, and rather implies that (1) the initial GCs were generally larger than the typical size of the presentday GCs, (2) the initially large GCs mostly shrank and/or disrupted as a result of the galactic tides, and (3) the initially small GCs expanded by twobody relaxation, and later shrank by the galactic tides. 

Publication at kias
NUMBER  K17024 
AUTHOR  Park, Changbom,Kim, Juhan 
TITLE  THE CHALLENGE OF THE LARGEST STRUCTURES IN THE UNIVERSE TO COSMOLOGY 
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JOURNAL  ASTROPHYSICAL JOURNAL LETTERS, 2012 
ABSTRACT  Large galaxy redshift surveys have long been used to constrain cosmological models and structure formation scenarios. In particular, the largest structures discovered observationally are thought to carry critical information on the amplitude of largescale density fluctuations or homogeneity of the universe, and have often challenged the standard cosmological framework. The Sloan Great Wall (SGW) recently found in the Sloan Digital Sky Survey (SDSS) region casts doubt on the concordance cosmological model with a cosmological constant (i.e., the flat Lambda CDM model). Here we show that the existence of the SGW is perfectly consistent with the Lambda CDM model, a result that only our very large cosmological Nbody simulation (the Horizon Run 2, HR2) could supply. In addition, we report on the discovery of a void complex in the SDSS much larger than the SGW, and show that such size of the largest void is also predicted in the Lambda CDM paradigm. Our results demonstrate that an initially homogeneous isotropic universe with primordial Gaussian random phase density fluctuations growing in accordance with the general relativity can explain the richness and size of the observed largescale structures in the SDSS. Using the HR2 simulation we predict that a future galaxy redshift survey about four times deeper or with 3mag fainter limit than the SDSS should reveal a largest structure of bright galaxies about twice as big as the SGW. 

Publication at kias
NUMBER  K17025 
AUTHOR  Park, Changbom,Kim, Juhan 
TITLE  A SECONDORDER BIAS MODEL FOR THE LOGARITHMIC HALO MASS DENSITY 
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JOURNAL  ASTROPHYSICAL JOURNAL, 2012 
ABSTRACT  We present an analytic model for the local bias of dark matter halos in a Lambda CDM universe. The model uses the halo mass density instead of the halo number density and is searched for various halo mass cuts, smoothing lengths, and redshift epochs. We find that, when the logarithmic density is used, the secondorder polynomial can fit the numerical relation between the halo mass distribution and the underlying matter distribution extremely well. In this model, the logarithm of the dark matter density is expanded in terms of log halo mass density to the second order. The model remains excellent for all halo mass cuts (from Mcut = 3 x 10(11) to 3 x 10(12) h (1) Mcircle dot), smoothing scales (from R = 5 h(1) Mpc to 50h(1) Mpc), and redshift ranges (from z = 0 to 1.0) considered in this study. The stochastic term in the relation is found to be not entirely random, but a part of the term can be determined by the magnitude of the shear tensor. 

Publication at kias
NUMBER  K17026 
AUTHOR  Park, Changbom,Kim, Juhan,Rossi, Graziano 
TITLE  THE NEW HORIZON RUN COSMOLOGICAL NBODY SIMULATIONS 
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JOURNAL  JOURNAL OF THE KOREAN ASTRONOMICAL SOCIETY, 2011 
ABSTRACT  We present two large cosmological Nbody simulations, called Horizon Run 2 (HR2) and Horizon Run 3 (HR3), made using 6000(3) = 216 billions and 7210(3) = 374 billion particles, spanning a volume of (7.200 h(1)Gpc)(3) and (10.815 h(1)Gpc)(3), respectively. These simulations improve on our previous Horizon Run 1 (HR1) up to a factor of 4.4 in volume, and range from 2600 to over 8800 times the volume of the Millennium Run. In addition, they achieve a considerably finer mass resolution, down to 1.25 x 10(11)h(1)M(circle dot), allowing to resolve galaxysize halos with mean particle separations of 1.2h(1)Mpc and 1.5h(1)Mpc, respectively. We have measured the power spectrum, correlation function, mass function and basic halo properties with percent level accuracy, and verified that they correctly reproduce the ACDM theoretical expectations, in excellent agreement with linear perturbation theory. Our unprecedentedly largevolume Nbody simulations can be used for a variety of studies in cosmology and astrophysics, ranging from largescale structure topology, baryon acoustic oscillations, dark energy and the characterization of the expansion history of the Universe, till galaxy formation science  in connection with the new SDSSIII. To this end, we made a total of 35 allsky mock surveys along the past light cone out to z = 0.7 (8 from the HR2 and 27 from the HR3), to simulate the BOSS geometry. The simulations and mock surveys are already publicly available at http://astro.kias.re.kr/HorizonRun23/. 

Publication at kias
NUMBER  K17033 
AUTHOR  Park, Changbom ,Kim, Juhan 
TITLE  ThreeDimensional Genus Topology of Luminous Red Galaxies 
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JOURNAL  Astrophysical Journal, 2009 
ABSTRACT  We measure the threedimensional genus topology of largescale structure using luminous red galaxies (LRGs) in the Sloan Digital Sky Survey and find it consistent with the Gaussian random phase initial conditions expected from the simplest scenarios of inflation. This studies threedimensional topology on the largest scales ever obtained. The topology is spongelike. We measure topology in two volumelimited samples: a dense shallow sample studied with smoothing length of 21 h 1 Mpc, and a sparse deep sample studied with a smoothing length of 34 h 1 Mpc. The amplitude of the genus curve is measured with 4% uncertainty. Small distortions in the genus curve expected from nonlinear biasing and gravitational effects are well explained (to about 1¥ò accuracy) by Nbody simulations using a subhalofinding technique to locate LRGs. This suggests that the formation of LRGs is a clean problem that can be modeled well without any freefitting parameters. This bodes well for using LRGs to measure the characteristic scales such as the baryon oscillation scale in future deep redshift surveys. 

Publication at kias
NUMBER  P08056 
AUTHOR  Park, Changbom,Choi, YunYoung,Kim, Juhan 
TITLE  A subhalogalaxy correspondence model of galaxy biasing 
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JOURNAL  ASTROPHYSICAL JOURNAL, 2008 
ABSTRACT  We propose a model for allocating galaxies in cosmological N body simulations. We identify each subhalo with a galaxy and assign luminosity and morphological type, assuming that the galaxy luminosity is a monotonic function of the host subhalo mass. Morphology is assigned using two simple relations between the subhalo mass and galaxy luminosity for different galaxy types. The first uses a constant luminosity ratio between early type (E/SO) and late type (S/Irr) galaxies at a fixed subhalo mass. The other assumes that galaxies of different morphological types but equal luminosity have a constant ratio of subhalomass. We made a series of comparisons of the properties of these mock galaxies with those of SDSS galaxies. The resulting mock galaxy sample is found to successfully reproduce the observed local number density distribution except in high density regions. We study the luminosity function as a function of local density, and find that the observed luminosity functions in different local density environments are overall well reproduced by the mock galaxies. Adiscrepancy is found at the bright end of the luminosity function of early types in the underdense regions and at the faint end of both morphological types in very high density regions. A significant fraction of the observed earlytype galaxies in voids seem to have undergone relatively recent star formation and become brighter. The lack of faint mock galaxies in dense regions may be due to the strong tidal force of the central halo, which destroys less massive satellite subhalos around the simulation. The mass to light ratio is found to depend on the local density in a way similar to that observed in the SDSS sample. We have found an impressive agreement between our mock galaxies and the SDSS galaxies in the dependence of central velocity dispersion on the local density and luminosity. 

Publication at kias
NUMBER  P07088 
AUTHOR  Park, Changbom,Kim, Juhan 
TITLE  Environment and mass dependencies of galactic lambda spin parameter: cosmological simulations and observed galaxies compared 
ARCHIVE  0712.0842 
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JOURNAL  MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 2008 
ABSTRACT  We use a sample of galaxies from the Sloan Digital Sky Survey (SDSS) to search for correlations between the lambda spin parameter and the environment and mass of galaxies. In order to calculate the total value of lambda for each observed galaxy, we employed a simple model of the dynamical structure of the galaxies, which allows a rough estimate of the value of lambda using only readily obtainable observables from the luminous galaxies. Use of a large volumelimited sample (upwards of 11 000) allows reliable inferences of mean values and dispersions of lambda distributions. We find, in agreement with some Nbody cosmological simulations, no significant dependence of lambda on the environmental density of the galaxies. For the case of mass, our results show a marked correlation with lambda, in the sense that lowmass galaxies present both higher mean values of lambda and associated dispersions, than highmass galaxies. These results provide interesting constrain on the mechanisms of galaxy formation and acquisition of angular momentum, a valuable test for cosmological models. 

Publication at kias
NUMBER  P06061 
AUTHOR  Park, Changbom,Choi, YunYoung,Kim, Juhan 
TITLE  Genus topology of structure in the Sloan Digital Sky Survey: Model testing 
ARCHIVE  astroph/0610762 
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JOURNAL  ASTROPHYSICAL JOURNAL, 2008 
ABSTRACT  We measure the threedimensional topology of largescale structure in the Sloan Digital Sky Survey (SDSS). This allows the genus statistic to be measured with unprecedented statistical accuracy. The sample size is now sufficiently large to allow the topology to be an important tool for testing galaxy formation models. For comparison, we make mock SDSS samples using several stateoftheart Nbody simulations: the Millennium run of Springel et al. ( 10 billion particles), the Kim & Park CDM models ( 1.1 billion particles), and the Cen & Ostriker hydrodynamic code models ( 8.6 billion cell hydro mesh). Each of these simulations uses a different method for modeling galaxy formation. The SDSS data show a genus curve that is broadly characteristic of that produced by Gaussian randomphase initial conditions. Thus, the data strongly support the standard model of inflation where Gaussian randomphase initial conditions are produced by random quantum fluctuations in the early universe. But on top of this general shape there are measurable differences produced by nonlinear gravitational effects and biasing connected with galaxy formation. The Nbody simulations have been tuned to reproduce the power spectrum and multiplicity function but not topology, so topology is an acid test for these models. The data show a meatball shift ( only partly due to the Sloan Great Wall of galaxies) that differs at the 2.5 sigma level from the results of the Millenium run and the Kim & Park dark halo models, even including the effects of cosmic variance. 

The HubbleDepth Survey
Title 
: The HubbleDepth Survey 
Journal 
: Revista Mexicanna de Astronomia y Astrofisica (Serie de Conferencias), 2007 
Publication at kias
NUMBER  P07085 
AUTHOR  Park, Changbom ,Kim, Juhan 
TITLE  The HubbleDepth Survey 
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JOURNAL  Revista Mexicanna de Astronomia y Astrofisica (Serie de Conferencias), 2007 
ABSTRACT  We have made a mock galaxy survey of the Hubble depth in an Nbody simulation of the ¥ËCDM model. To populate galaxies in the particle distribution, we identify selfbound and stable halos by a halo finding method in the real and lightcone space. We assume that each halo contains only one galaxy with brightness monotonically proportional to the halo mass to match the mass function with the galaxy luminosity function obtained in the Sloan Digital Sky Survey (SDSS). After applying the K and evolutionary corrections to the observed luminosity of the mock galaxies, we have made mock redshift surveys under various observational constraints. In particular, we propose a new redshift survey, named the Hubble Depth Survey, which is magnitudelimited down to r* = 22 and reaching the Hubble distance, dH = 3000 h1 Mpc. 

Publication at kias
NUMBER  P05060 
AUTHOR  Park, C,Kim, J 
TITLE  A new halofinding method for Nbody simulations 
ARCHIVE  astroph/0401386 
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JOURNAL  ASTROPHYSICAL JOURNAL, 2006 
ABSTRACT  We have developed a new halo finding method called the physically self bound ( PSB) group finding algorithm, which is designed to identify halos located in crowded regions. To demarcate subhalo regions, we introduce the tidal constraint combined with the self boundedness check. In order to reduce overload and parallel finding, we first divide whole simulation particles into many local particle groups using two distinct methods. One adopts a density mesh used to group particles in connected local overdense cells, and the other applies the friends of friends ( FoF) algorithm with a linking length ( l(loc)) 0.3 times the mean particle separation. Then we divide each local particle group into several subgroups enclosed by numerous density levels and identify tidally stable and self bound subhalos around density peaks. Subhalo finding is done on a fine mesh with cell size equal to twice the force resolution. Particles in a density shell that surrounds only one density peak form a subhalo candidate. Particles located in the outer remaining density shells that surround more than one peak become member candidates of one of the subhalos. We determine the membership using the tidal boundary constraint. We have found that the mass function is very insensitive to l(loc) when l(loc) >= 0: 3. At least 40% of subhalos do not seem to collapse to the most massive subhalo in a FoF group when the Press & Schechter collapse model is applied to measure the size of a collapsed structure in the Lagrangian space. We have applied our halo finding method to a 1024(3) particle simulation in a Lambda CDM model and compare the halo mass functions with those previously found in the literature. 

Publication at kias
NUMBER  P05049 
AUTHOR  Park, C,Choi, YY,Kim, J 
TITLE  Topology analysis of the Sloan Digital Sky Survey. I. Scale and luminosity dependence 
ARCHIVE  astroph/0507059 
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JOURNAL  ASTROPHYSICAL JOURNAL, 2005 
ABSTRACT  We measure the topology of volumelimited galaxy samples selected from a parent sample of 314,050 galaxies in the Sloan Digital Sky Survey (SDSS), which is now complete enough to describe the fully threedimensional topology and its dependence on galaxy properties. We compare the observed genus statistic G(v(f)) to predictions for a Gaussian random field and to the genus measured for mock surveys constructed from new largevolume simulations of the Lambda CDM cosmology. In this analysis we carefully examine the dependence of the observed genus statistic on the Gaussian smoothing scale RG from 3.5 to 11 h(1) Mpc and on the luminosity of galaxies over the range 22.50 < Mr < 18.5. The void multiplicity A(V) is less than unity at all smoothing scales. Because A(V) cannot become less than 1 through gravitational evolution, this result provides strong evidence for biased galaxy formation in lowdensity environments. We also find clear evidence of luminosity bias of topology within the volumelimited subsamples. The shift parameter Delta v indicates that the genus of brighter galaxies shows a negative shift toward a meatball (i.e., cluster dominated) topology, while faint galaxies show a positive shift toward a bubble (i.e., void dominated) topology. The transition from negative to positive shift occurs approximately at the characteristic absolute magnitude Mr* = 20.4. Even in this analysis of the largest galaxy sample to date, we detect the influence of individual largescale structures, as the shift parameter Delta v and cluster multiplicity A(C) reflect (at similar to 3 sigma) the presence of the Sloan Great Wall and an Xshaped structure that runs for several hundred megaparsecs across the survey volume. 

Publication at kias
NUMBER  P05027 
AUTHOR  Park, C,Kim, J 
TITLE  Effects of gravitational evolution, biasing, and redshift space distortion on topology 
ARCHIVE  astroph/0503584 
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JOURNAL  ASTROPHYSICAL JOURNAL, 2005 
ABSTRACT  We have studied the dependence of topology of largescale structure on tracer, gravitational evolution, redshift space distortion, and cosmology. A series of large Nbody simulations of the Lambda CDM and SCDM models that have evolved 1.1 or 8.6 billion particles are used in the study. Evolution of the genus statistic, used as a topology measure, from redshift 8 to 0 is accurately calculated over a wide range of smoothing scales using the simulations. The tracers of largescale structure considered are the cold dark matter (CDM), biased peaks in the initial density field, dark halos, and galaxies populating the dark halos in accordance with a halo occupation distribution ( HOD) model. We have found that the effects of biasing, gravitational evolution, and initial conditions on topology of largescale structure are all comparable. The redshift space distortion effects are relatively small down to about 5 h(1) Mpc for all tracers except for the highthreshold part of the genus curve. The gravitational effects are found to be well modeled by analytic perturbation theory when the CDM distribution is considered. But the direction of gravitational evolution of topology can be even reversed for different tracers. For example, the shift parameter of the genus curve evolves in opposite directions for matter and HOD galaxies at large scales. At small scales, there are interesting deviations of the genus curve of dark halos and galaxies from that of matter in our initially Gaussian simulations. The deviations should be understood as due to combined effects of gravitational evolution and biasing. This fact gives us an important opportunity: topology of largescale structure can be used as a strong constraint on galaxy formation mechanisms. At scales larger than 20 h(1) Mpc all the above effects gradually decrease. With good knowledge of the effects of nonlinear gravitational evolution and galaxy biasing on topology, one can also constrain the Gaussian random phase initial conditions hypothesis to high accuracy. 

Publication at kias
NUMBER  K17034 
AUTHOR  Kim, Juhan 
TITLE  GOTPM: a parallel hybrid particlemesh treecode 
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JOURNAL  New Astronomy, 2003 
ABSTRACT  

Publication at kias
NUMBER  K17035 
AUTHOR  Park, Changbom ,Kim, Juhan 
TITLE  Diffuse Dark and Bright Objects in the Hubble Deep Field 
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JOURNAL  The Astrophysical Journal, 1998 
ABSTRACT  In the Hubble Deep Field (HDF) we have identified candidate regions where primordial galaxies might be forming. These regions are identified from negative or positive peaks in the difference maps obtained from the HDF maps smoothed over 0.8 and 4. They have apparent V magnitudes typically between 29 and 31 (missing flux below the local average level for the dark objects) and are much fainter than the nearby L* galaxies. The identified objects are shown to be real in two ways. First, the crosscorrelations of these peaks detected in different filters are strong. The bright objects have the crosscorrelation lengths of about 0.3. Second, their autocorrelation functions indicate that these faint diffuse objects are selfclustered. Furthermore, the autocorrelation function for the highredshift starburst subset of bright objects selected by color, has an amplitude significantly higher than that of the total sample. The subset of objects, dark in the F450W and F606W bandpasses, but bright in F814W, also shows stronger correlation compared to the whole dark sample. This further supports that our samples are indeed physical objects. The amplitude and slope of the angular correlation function of the bright objects indicate that these objects are ancestors of the present nearby bright galaxies. It is shown that the data reduction artifacts cannot be responsible for our sample.
We have inspected individual bright objects and noted that they have several tiny spots embedded in extended backgrounds. Their radial light distributions are diverse and quite different from those of nearby bright galaxies. They are likely to be the primordial galaxies at high redshifts in the process of active star formation and merging.
The dark objects in general appear smooth. Our subset of the dark objects is thought to be the ``intergalactic dark clouds blocking the background farUV light (at the rest frame) at high redshifts instead of empty spaces between the first galaxies at the edge of the universe of galaxies.
Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, under NASA contract NAS526555. 
 Sep 10 – Now Research Professor at Korea Institute for Advanced Study
 Sep 09 – Aug 10 Research Professor at Kyung Hee University
 Sep 08 – Aug 09 Visiting Postdoctor to Canadian Institute for Theoretical Astrophysics
 Nov 04 – Aug 08 Research Fellow, Korea Institute for Advanced Study
 Sep 03 – Aug 04 Postdoctoral Fellowship, Korea Astronomical Observatory
 Office:7309 / TEL) 8229583795 /
 Center for Advanced Computation, Korea Institute for Advanced Study
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